Mine ventilation structures such as overcasts and undercasts are widely used in mines to prevent mixing of forced (or induced) ventilation air flowing in one passage with forced (or induced) ventilation air flowing in another passage at an intersection of those passages. Generally, an overcast comprises a tunnel (e.g., made of two sidewalls and a deck) erected in one of the passages and extending through the intersection with the other passage. The tunnel blocks communication of air between the passages at the intersection, but permits air in one of the passages to flow through the tunnel and permits air in the other passage to flow through the intersection in a space between the top of the tunnel and the deck. Additional details relating to the construction and operation of overcasts are provided in our U.S. Pat. Nos. 5,412,916, 6,264,549, 5,466,187, 7,182,687 and 7,232,368, all of which are incorporated herein by reference. An undercast is similar to an overcast, but the tunnel is constructed adjacent the roof of the intersection (e.g., the sidewalls and deck are inverted and suspended above the floor). Air in one of the passages flows through the tunnel of the undercast and the air in the other passage flows through the intersection in a space between the bottom of the tunnel and the floor of the intersection.
Ventilation structures are desirably relatively lightweight and relatively small so that they are easy to assemble and do not unnecessarily restrict airflow through the passage.